EXAMINATION or Till': KYKHM.L KK1 



plying ('S]K'ci:illy the sphincter muscle. Tin- conn al nerves form an annular plexus 

 near the limbus. from which a few twigs proceed to the sclerotic and conjunctiva, 

 while most of the offsets enter and run radially in the corneal stroma, branching 

 and anastomosing so as to form :i plexus. The nerves entering the cornea are 

 about sixty in number, each containing from two to twelve non-medullatcd nerve- 

 filires (page 1)72). 



Blood-vessels of the eyeball. The ocular tissues receive blood from two sets 

 of vessels, \-'[7.., the retinal and the ciliary arteries. 



1. The arteria centralis retinae either conn s direct from the ophthalmic artery, 

 or from one of its branches near the apex of the orbit. Kntering the optic nerve 

 twenty millimetres or less behind the globe, it runs forwards in its axis to the end of 

 the nerve-trunk, and then divides into branches which run in the inner layers of the 

 retina, and divide dichotomously as they radiate towards the equator. The smaller 

 branches lie more deeply in the retina, but none penetrate into the nerve-epithelium, 

 so that the fovea centralis is non-vascular. In the retina, the branches of the cen- 

 tral artery do not communicate with any other arteries, but while still in the optic 

 nerve fine communications take place between this artery and neighbouring vessels. 

 Thus (a) minute twigs from it. which help to nourish the axial part of the nerve, 

 communicate with those running in the septa derived from the pial sheath. Again, 



FIG. 730. SURFACE OF CHORIOID AND IRIS EXPOSED BY REMOVAL OF SCLEROTIC AND 

 CORNEA, SHOWING DISTRIBUTION OF BLOOI>-\ KSSF.I.S AND NERVES. 



(Twice natural size. After Zinn.) 



SURFACE OF CHORIOID IN CILIARY 

 REGION . 



Vena rorticota 



^B Hv^^vv^SW^kS^E^^H^H 



A CILIARY KERVE- 



V ^^ Lmgpoikrtor Hilary artery 



FLAP OF SCLEROTIC. REFLECTED 



as the nerve passes through the sclerotic, it is surrounded by a vascular ring (circle 

 of Haller), formed of fine branches derived from the short posterior ciliary arteries; 

 fine twigs passing inwards from this ring to the optic nerve join the vessels of the 

 pial sheath, and (6) an indirect communication is thus brought about between the 

 retinal and ciliary vessels. Finally, as the nerve passes through the chorioid, there 

 is (c) a direct connection between these two sets of vessels, the capillary network of 

 the optic nerve being here continuous with the chorio-capillaris. Not infrequently, 

 a branch from a short posterior ciliary artery pierces the optic papilla, and then 

 courses over the adjoining retina (a cilio-retinal artery), supplying the latter in part 

 in place of the central artery. 



The vena centralis retinas returns the blood of the corresponding artery. 



2. The ciliary system of blood-vessels (pages 536, 537, and 662). There are 

 three sets of arteries belonging to this system, all derived directly or indirectly 

 from the ophthalmic artery. 



(1) Short posterior ciliary arteries, twelve to twenty in number, pierce the 

 sclerotic round the optic nerve entrance, and are distributed in the chorioid. liefore 

 entering the eyeball, small twiirs are given off to the adjoining sclerotic and to the 

 dural sheath of the optic nerve. 



(2) Two long posterior ciliary arteries, piercing the sclerotic further from the 



